Question

What kind of magmas produce violent volcanic eruptions? A) those rich in silica B) those that are fluid C) those forming shield volcanoes D) those rich in iron

Short answer

A) those rich in silica

Step-by-step solution

Identify Characteristics of Magmas That Affect Eruptions

Magma composition plays a crucial role in determining the nature of volcanic eruptions. Particularly, the viscosity of magma is a key factor. Magmas rich in silica generally have higher viscosity, making them more explosive because gases cannot escape easily.

Analyze Each Option

Let's examine each option: A) Magmas rich in silica have high viscosity and often lead to explosive eruptions due to trapped gases. B) Fluid magmas (low viscosity) allow gases to escape easily, resulting in milder eruptions. C) Magmas forming shield volcanoes are generally basaltic, which is fluid and produces gentler eruptions. D) Magmas rich in iron are typically basaltic, associated with fluid lava flows and less violent eruptions.

Determine the Correct Answer

Based on the analysis, option A is the correct answer. Magmas rich in silica lead to violent volcanic eruptions.

Key concepts

Silica-rich Magmas

Silica-rich magmas are predominantly composed of silicon dioxide (SiO₂). These magmas are known for their high silica content, which significantly influences their properties and behavior during volcanic eruptions.

• Silica-rich magmas have a high viscosity, meaning they are thick and sticky, preventing gases from escaping easily. This results in a buildup of pressure within the magma chamber.
• The trapped gases can include water vapor, carbon dioxide, and sulfur compounds, which contribute to the explosive potential of these magmas.
• The high silica content also leads to the formation of rhyolite and dacite rocks, which are typical of explosive volcanic eruptions such as those at Mount St. Helens and Mount Vesuvius.

Silica-rich magmas are often associated with highly explosive volcanic eruptions. The great energy released during these events can result in significant volcanic hazards including pyroclastic flows and volcanic ash clouds.

Magma Viscosity

The term **viscosity** refers to the resistance of a liquid to flow. In the context of volcanic eruptions, magma viscosity is a crucial element that determines the style and intensity of an eruption. For magmas:

• High viscosity means a thick, slow-flowing lava, commonly seen with silica-rich magmas. The high resistance to flow makes it difficult for volcanic gases to escape, which increases explosive potential.
• Low viscosity indicates a runnier, fast-flowing lava, usually found in basaltic magmas. This type of magma allows gases to escape more easily, leading to less explosive eruptions.

Understanding magma viscosity is key to predicting the eruption behavior of a volcano. High-viscosity magmas can result in more catastrophic eruptions, whereas low-viscosity magmas tend to create steady lava flows.

Volcano Types

Volcanos are categorized into several types based on their morphology, eruption style, and magma composition. Here are the primary types:

• **Shield Volcanoes**: These are large, broad volcanoes with gentle slopes, formed from basaltic magmas. The low viscosity of basaltic lava allows it to flow over long distances, producing non-explosive, fluid eruptions. Mauna Loa in Hawaii is a prime example.
• **Composite Volcanoes (Stratovolcanoes)**: Characterized by steep slopes, these volcanoes form from alternating layers of lava and ash. They are generally associated with silica-rich magmas and are prone to explosive eruptions. Mount St. Helens is a notable composite volcano.
• **Cinder Cone Volcanoes**: These are smaller, steep-sided volcanoes formed by the accumulation of volcanic debris. They usually erupt basaltic lava but can vary in explosiveness.

Each volcano type exhibits distinct eruption characteristics based on the magma it produces, which directly results in different volcanic hazards.